1. Field of the Invention
The present invention relates to an optical diffraction grating suitable for use, for example, as a wavelength multiplexer/demultiplexer in an optical telecommunications system.
2. Related Art
Bulk optic diffraction gratings are well known, and it has previously been proposed to use such gratings as passive multiplexers/demultiplexers in optical networks employing wavelength division multiplexing (WDM). The use of bulk-optic components tends however to result in high packaging and maintenance costs. Accordingly, while the use of such components might be feasible if wavelength multiplexing/demultiplexing was to be confined to a few core switches, bulk optic components are not suitable for more widespread use in a network. Current interest in WDM centres on its use in local access networks in combination with optical time division multiplexing (OTDM) for longer links in the network. There remains a need therefore for a grating which is sufficiently robust and inexpensive to be used in local access loops throughout a network, and possibly to be present in each subscriber terminal.
The paper by Poguntke and Soole, "Design of A Multistripe Array Grating Integrated Cavity (MAGIC) Laser", Journal of Light Wave Technology, Vol. 11 No. 12 December 1993, discloses a grating formed in an InP-based planar waveguide structure. The grating is defined using photolithography and dry etched using, for example, chemically assisted ion-beam etching, to form a stepped wall extending perpendicularly through the planar waveguide. The grating is then metallised in order to improve its reflectivity. This structure, however, offers only limited angular dispersion, and so is not able to accommodate many wavelength channels without becoming unacceptably large.